The present invention relates to drive tractors which are widely used in printers and other devices for handling webs such as continuous perforated paper, individual sheet paper, tape and the like, of paper, foil, laminates and other sheet-like material, all collectively referred to hereinafter as "web material".
Generally such devices employ a belt which will engage the web material either by pins or projections on the upper surface of the belt extending into perforations in the web material or by friction or "clamping" of the web material between the upper surface of the belt and a cover or guide thereabove. Such devices are in widespread use for the applications indicated above and are particularly burgeoning in usage for paper handling in printers for computers, word processing and duplicating apparatus.
Among the problems which have been encountered in the use of such tractors are aberrations in cross section in the drive shaft upon which the tractors are mounted, bowing or other deflection of the shafts in the distance between the pair of tractors mounted thereon and cooperating to drive the web material, and deflection of the shafts during operation. Tolerances in the shaft receiving apertures of the tractors, or in the clamping means, or both, have been proposed to accommodate the shaft irregularities discussed above. Hubbard U.S. Pat. No. 4,129,239 granted Dec. 12, 1978 discloses a tractor in which the clamping mechanism for the support shaft is a part of a pillow block on the chassis which may "float" in spacing relative to the drive shaft.
Particularly in high speed web handling devices, it is important that the spacing between the paper guiding surfaces and in fact between the shaft receiving portions be maintained substantially constant and that the tractor not move in the direction of web travel. This spacing and fixed positioning is critical to uniform advance of the web material, particularly when the drive tractor is bidirectional, i.e., movable backward and forward abruptly. Thus, there is need to accommodate aberrations in the shafts and variations in spacing between their axes while maintaining constant the distance between axes of the paper guide surfaces and the axes of the shaft receiving apertures and while avoiding any movement of the tractor relative to the printer or other device on which it is mounted. If not, undesirable variations in line spacing during printing of the web may occur.
In the aforementioned copending application of Alan F. Seitz, there is disclosed a tractor assembly employing a drive pulley with resiliently deflectable means gripping the drive shaft and deflecting to accommodate aberrations in the drive shaft and variations in spacing between the drive and support shafts. The tractors of that application provide a significant amount of accommodation, but there is a limitation determined by the range of deflection of the deflectable means claimed therein.
It is an object of the present invention to provide a novel drive tractor for web material which readily accommodates aberrations in the drive shaft cross section and relatively large deviations in spacing between the axes of the drive and support shafts while maintaining essentially constant the axial spacing of the web guide surfaces and of the shaft receiving apertures in the tractor chassis.
It is also an object to provide such a drive tractor which may be fabricated readily and relatively economically and which will enjoy long life in operation.
Another object is to provide such a tractor which remains fixed on the printer or drive device upon which it is mounted so as to avoid variations in tension on the web material being driven thereby and line spacing in the web.
A further specific object is to provide such a tractor wherein the drive shaft may deflect within the drive pulley without effecting variation in spacing of the drive pulley relative to the other pulley or the support shaft receiving aperture.